1. Field of the Invention
The present invention relates to retroreflective raised pavement markers that are used for traffic markings and delineation, and more particularly to a durable raised pavement marker of high apparent modulus which possesses a high flexural modulus and impact strength to resist vehicle impact.
2. Related Art
Raised pavement markers are widely used as highway traffic markings for providing road lane delineation. One type of raised pavement marker is a retroreflective marker having a shell housing which is filled with a hard and brittle potting compound. These markers tend to sustain a high rate of breakage and shattering upon cyclic vehicle impact. However, at least one manufacturer of these markers has attempted to improve the durability of the housing. For instance, U.S. Pat. No. 5,340,231 to Steere et al. (assigned to the Stimsonite Corporation), teaches the use of chopped glass fiber reinforced block terpolymer acrylic-styrene-acrylonitrile for molding the housing but still fills the housing cavity with a rigid epoxy compound.
The use of high impact strength plastic material (i.e., a plastic material having an impact strength of higher than 1 foot-pound/inch as defined and measured by ASTM D1822) for making the housing has been practiced by the assignee of the present application, the Minnesota Mining and Manufacturing Company, Inc. ("3M") since the mid-1980's. Such use of high impact resistant material is disclosed in U.S. Pat. No. 4,875,798 to May (assigned to "3M"), and resulted in the commercialization of the high performance 3M model 280, SP280, 240, and SP240 markers.